Apparatus for coating granules with a thin dense layer

ABSTRACT

A method and apparatus for applying a thin, protective coating to individual granules by generating a liquid screen of the coating material at one end of a vertical shaft, passing the granules down through the screen and onto baffles disposed in the lower portion of the shaft. The liquid will pass down the walls of the shaft and onto the baffles and become entrained with the granules impinging thereon. A rotary drum is provided adjacent the discharge end of the shaft to intimately mix the materials passing out of the shaft.

United States Patent De Wit et a1.

APPARATUS FOR COATING GRANULES WITH A THIN DENSE LAYER lnventors: WillemDe Wit, Heerlen; Petrus F. A. M. Hendriks, Sittard, both of NetherlandsAssignee: Stamicarbon N.V.

Filed: Sept. 29, 1970 Appl. No.: 76,439

US. Cl. ..ll8/303, 118/19, 118/417, 117/100 Int. Cl. ..B05b 17/00, 1305c5/00 Field ofSearch.....l18/303, 19, 417; 117/100 A, 117/100 B, 100CReferences Cited UNITED STATES PATENTS Erickson..... ....1 18/303 XMahoney ....118/303 X Tadema et a1. ..1l 8/303 X [451 Feb. 13, 19733,518,967 7/1970 Horrocks ..118/303 2,561,392 7/1951 Marshall ..117/100A 3,101,040 8/1963 Lanz ..118/303 X 2,993,469 7/1961 Tarpley, Jr. et a1...118/303 3,112,220 11/1963 Heiser, Jr. et a1 ..118/303 X 1,473,165ll/1923 Steigmeyer ..1l8/417 X 3,198,655 8/1965 Gisiger ..118/303 X3,110,626 11/1963 Larson et a1 ..118/303 Primary Examiner.lames Kee ChiAtlorneyCushman, Darby & Cushman [57] ABSTRACT A method and apparatusfor applying a thin, protective coating to individual granules bygenerating a liquid screen of the coating material at one end of avertical shaft, passing the granules down through the screen and ontobaffles disposed in the lower portion of the shaft. The liquid will passdown the walls of the shaft and onto the baffles and become entrainedwith the granules impinging thereon. A rotary drum is provided adjacentthe discharge end of the shaft to intimately mix the materials passingout of the shaft.

4 Claims, 3 Drawing Figures PATENTEDFEB 13 I973 SHEET 2 CF 2 APPARATUSFOR COATING GRANULES WITH A THIN DENSE LAYER The present inventionrelates to a process of coating granules with a thin dense, protectivelayer, initially applied in the liquid state. In particular, theinvention relates to the coating of fertilizer granules, e.g. urea withan oily substance, which functions to reduce the lowering of the rollingtendency and the deliquescence of the granules owing to excessivemoisture absorption from the air during transport and storage of thefertilizer. This moisture absorption may be largely suppressed bybagging the granules immediately after their production instead oftransporting them in bulk. However, from an economic point of view it isgenerally more efficient to store and transport the granules in bulk,after which bagging can be done as desired. The coating substanceemployed here, may consist, for instance, of 75 percent mineral oil, 15percent paraffin, and percent fat and is maintained at a temperature ofapproximately 50-60 C during its application, while the granules aremaintained in a temperature range between 30-50C. After the application,the paraffin crystallizes. The oil is gradually soaked up by thegranules, and the fat prevents chemical reaction of the paraffin withthe granule. The coating should not be much thicker than 0.3a, forreasons of economy and in order that the guaranteed nitrogen content maybe maintained. The coating layer should satisfy high qualityrequirements and entirely encompass each granule. Even a very smallopening in the coating layer may admit sufficient moisture to destroythe effect of the coating completely.

Many attempts have been made to produce a satisfactory coating, eg byspraying or immersion followed by centrifugation, but so far it had beenimpossible to obtain a coating of uniform and very small thickness whichcompletely envelops the granules.

The present invention provides a solution which avoids the disadvantagesof the prior art. More specifically, a screen of liquid is caught on oneor more obliquely disposed walls or baffles, and the granules fallthrough the screen-and onto the baffles and entrain the liquid flowingdown along the baffles. The granules entrain some of the liquid of thescreen in falling onto the baffles, and roll down along each baffle. Theliquid screen is so controlled that all of the liquid not contacted bythe granules during their fall through the screen is entrained by thegranules rolling down along each baffle, so that little or no liquiddrops from the bottom edge of these baffles. On account of the rolling Imotion, the liquid is spread uniformly over the surface ofthe-granules.-

In a preferred embodiment, the liquid screen is disposed at an angle tothe vertical, and the granules are allowed to fall freely therethrough.In practice, this arrangement is both more effective as well as simplerto implement than a vertical liquid screen.

In many cases, it is desirable to intimately mix the granules collectedat the bottom edge of thewalls. Owing to the friction among-thegranules, the spreading out of the layer of coating material, theviscosity of which increases through crystallization, is completed bysuch mixing.

The invention also relates to an apparatus for carrying into effect theprocess herein disclosed which comprises a vertical shaft with a feedopening at the top and a discharge opening at the bottom, in the upperpart of which shaft is mounted a sprayer of the type capable ofgenerating a liquid screen. On different levels in the shaft, there areprovided obliquely disposed baffles, which, together, are capable ofintercepting all of the liquid and all of the granules passing down theshaft.

In the preferred embodiment, the sprayer consists of a cylindricalchamber placed axially in the vertical shaft, with a tangential feedopening and an axial discharge opening, to which chamber is tangentiallyconnected a tube for the discharge of the material outside the shaft,this tube being provided with an adjustable valve. With thisconstruction, the force with which the liquid forming the initiallyconical screen leaves the axial discharge opening is not influenced bythe amount of liquid forming the screen. This amount is controlled bymeans of the valve. When the valve is fully open, the liquid willby-pass the axial discharge opening.

It is preferrable that the vertical shaft have a double wall, the spacebetween the two walls extending to below the oblique walls and beingprovided with connections for the supply and discharge of a heatingagent. If this agent has a temperature of, say, C, the liquid isprevented from solidifying on the wall and forming encrustations there,which might clog up the shaft.

The vertical shaft may discharge at its lower end into a mixing vesselconsisting of a sloping rotary drum with a supply opening in the top endface and a discharge opening in the bottom end face, with vanes providedinternally therein. The quantity of the granules in this drum is more orless constant and during this stay the granules are well mixed and thecoating material is properly distributed on their surface by thefriction.

The drum is provided with vanes spaced from the drum wall and sodisposed that, when the vanes pass through the granular content, thevanes emerge in a plane nearly coincident with the surface of thegranular mass. Then ensures a proper mixing of the innermost parts ofthe drum contents, and prevents the granules falling freely during themixing operation, which could cause breakage of the coating. Breakage ofthe coated granules when the liquid already has solidified more or less,when not followed by intimate mixing, will result in granules (or ratherpieces of granules) without a completely enveloping coating beingdelivered, which it is an object of this invention to prevent. Theruptured faces of granules breaking in the shaft, however, will stillbecome covered with a liquid layer, due to'the friction of the granulesand creep of the liquid in the lower portion of the shaft and mixing inthe drum.

The invention will be elucidated with reference to the drawing, whichshows one embodiment. In these drawings, 7

FIG. 1 is a vertical section of a vertical shaft with a sprayer andmixing drum according to the invention;

FIG. 2 is a vertical section of a sprayer on a larger scale; and

FIG. 3 is a cross section of the mixing drum along the line III-Ill inFIG. 1, drawn to a larger scale.

In FIG. 1, a vertical shaft 1 is represented having a feed opening 2 forfreely falling granules, a sprayer, 3, oblique wall sections or baffles4 and 5, a discharge opening 6 emptying-into a feed opening 7 of a drum8 with a discharge opening 9.

As seen in FIG. 2, the sprayer 3 has a bipartite cylindrical chamber 10,11, with a tangential feed opening 12 in the top compartment 10,connected to a feed pipe 13, and a tangential discharge opening (notshown) in the bottom compartment 11, to which is connected a dischargepipe 14. Appropriate means (not shown) of conventional design areprovided to maintain the temperature of the liquid between 50-60C as itis fed to sprayer 3.

The discharge pipe 14 is equipped with an adjustable valve 15. Thesprayer 3 is provided with a nozzle 16 for axial discharge. The liquidwhich, from the feed pipe 13, tangentially enters the top cylindricalchamber compartment is here set in rapid rotation, so that a ring ofliquid is formed. If the valve is fully open, the liquid passes throughthe bottom compartment 11 to leave the sprayer 3 by discharge pipe 14.In that case no liquid issues through the nozzle 16. According to thedegree valve 15 restricts flow, less liquid will be discharged throughpipe 14, and the ring ofliquid in the chamber compartment will increaseso that it will soon fill up compartment 11 completely, and the liquidwill commence issuing through the nozzle 16. Due to the centrifugalforce generated, in the cylindrical chambers 10 and 11 a conical screen17 of liquid will be formed. The force with which the liquid is flungaway is independent of the position of valve 15, and is in partdetermined by the pressure of the liquid in feed pipe 13. The amount ofliquid forming the liquid screen can be accurately controlled as neededby means of valve 15.

The oblique baffle sections 4 and 5 in the shaft 1 each extend inward tothe center of the shaft. Together, the baffle sections 4 and 5 thereforereceive all granules falling down through the liquid screen 17.Likewise, all liquid not caught bythe falling granules is intercepted bythe baffles 4 and 5. The valve 15 is so adjusted that the-desired ratiobetween liquid and solid material is obtained, e.g. by means of asuitable ratio control mechanism of conventional design. All liquidflowing down along the walls of the shaft onto baffles 4 and 5 is thusentrained by the granules rolling down along the walls.

The section of the shaft 1 containing the sloping baffles sections isprovided with a double wall, forming a space -18 for a heating medium,e.g. water, oil, or the like. To enable this medium to circulate, thespace is provided with connections 19 and 20 for supply and discharge.The space 18 extends to below the level of the oblique wall sections 4and 5. Ordinarily, the heating medium is maintained at a temperature ofabout 100C to warm the granules and prevent the liquid from solidifyingon the walls of the shaft 1.

The warm granules, now coated with a layer of liquid, leave the shaftthrough the discharge opening 6,

which connects to the feed opening 7 in the top end face of the slopingrotary drum 8. In this drum the granules are intimately mixed, duringwhich process the coating'gets more and more viscous and is, up to thelast moment, spread out by the granules rubbing against each other.

The granules are first caught by a few entrance vanes 21, which ensuresa uniform feed into the first (22) of three compartments 22, 23 and 24.These compartments are identical, but are turned through an angle of 20with respect to each other. Between the compartments there are bushingrings 25. The three compartments are rotated about their longitudinalaxis by suitable power means of conventional design.

The cross section of compartment 23, shown in FIG. 3, shows six mixingvanes 26, spaced from the wall of the drum 8. In addition, there are l2braking strips 27 on the wall. These narrow strips provide that if thereis only little material in the drum, the contents 28 will be entrainedover a desired length of arc. If no mixing vanes 26 were used, thecontents would keep rolling down, so that the innermost particles of thecontents, the core, would not be affected by the mixing or would not bemixed sufficiently. The core is stirred up by the vanes 26. The positionof the vanes 26 is such that when the granular quantity 28 is largeenough to ensure contact of the vanes with the granules, the granules donot drop down at all or, if they do, drop from a very small height only,when the vanes pass through the content 28 and 29.

When the granules issue from the discharge opening 9, they are ready forstorage and transport.

The invention is not restricted to the embodiment here described. Forinstance, the mixing drum and the sprayer may be modified in manydetails without departing from the principles of the invention. Theinvention provides a simple and effective solution for the problem posedwhich makes a valuable contribution towards the realization of betterbulk transportation.

What is claimed is:

1. Apparatus for coating material in the form of granules with a liquidto form a thin, substantially uniform protective layer on each of thegranules, said apparatus comprising a generally vertically disposedshaft having an interior surface, a material feed opening at the upperend of said shaft, a material discharge opening at the lower end of saidshaft, means disposed adjacent said upper end of said shaft fordischarging said liquid in the form of a liquid screen into said shaft,said liquid discharging means including a first and a second cylindricalchamber each disposed axially in said shaft and having a tangentialinlet and an axial outlet, each of said cylindrical chambers beingdisposed about a common axis, said axial outlet comprising'a centrallylocated discharge nozzle having a portion located in said first chamberand a portion located in said second chamber, said first chamber havinga circular aperture for establishing flow communication with said secondchamber, said circular aperture having a surface surrounding andradially spaced from said portion of said discharge nozzle locatedtherein, one of said two chambers being connected to a liquid feedconduit, the other being connected to a liquid discharge conduit havingadjustable valve means therein, a plurality of baffled disposedobliquely with respect to said interior surface of said shaft so that,in combination, said baffles will be contacted by all liquid and allgranules passing down through said shaft.

2. The apparatus as claimed inclaim 1, wherein said vertical shaft isprovided with a double wall extending along the axial length of saidshaft from a point proximate the liquid discharging means to a pointbelow said baffles, said double wall being provided with conduit meansfor the supply and discharge of a heating agent.

3. The apparatus as claimed in claim 1, wherein a mixing vessel isprovided to receive the material discharged from the lower end of saidvertical shaft,

mass of material contained in said drum so that upon passing throughsaid material, in the course of the rotation of said drum, said vaneswill emerge from said material in a plane that is substantially parallelto the plane of the surface of said material.

1. Apparatus for coating material in the form of granules with a liquidto form a thin, substantially uniform protective layer on each of thegranules, said apparatus comprising a generally vertically disposedshaft having an interior surface, a material feed opening at the upperend of said shaft, a material discharge opening at the lower end of saidshaft, means disposed adjacent said upper end of said shaft fordischarging said liquid in the form of a liquid screen into said shaft,said liquid discharging means including a first and a second cylindricalchamber each disposed axially in said shaft and having a tangentialinlet and an axial outlet, each of said cylindrical chambers beingdisposed about a common axis, said axial outlet comprising a centrallylocated discharge nozzle having a portion located in said first chamberand a portion located in said second chamber, said first chamber havinga circular aperture for establishing flow communication with said secondchamber, said circular aperture having a surface surrounding andradially spaced from said portion of said discharge nozzle locatedtherein, one of said two chambers being connected to a liquid feedconduit, the other being connected to a liquid discharge conduit havingadjustable valve means therein, a plurality of baffled disposedobliquely with respect to said interior surface of said shaft so that,in combination, said baffles will be contacted by all liquid and allgranules passing down through said shaft.
 1. Apparatus for coatingmaterial in the form of granules with a liquid to form a thin,substantially uniform protective layer on each of the granules, saidapparatus comprising a generally vertically disposed shaft having aninterior surface, a material feed opening at the upper end of saidshaft, a material discharge opening at the lower end of said shaft,means disposed adjacent said upper end of said shaft for dischargingsaid liquid in the form of a liquid screen into said shaft, said liquiddischarging means including a first and a second cylindrical chambereach disposed axially in said shaft and having a tangential inlet and anaxial outlet, each of said cylindrical chambers being disposed about acommon axis, said axial outlet comprising a centrally located dischargenozzle having a portion located in said first chamber and a portionlocated in said second chamber, said first chamber having a circularaperture for establishing flow communication with said second chamber,said circular aperture having a surface surrounding and radially spacedfrom said portion of said discharge nozzle located therein, one of saidtwo chambers being connected to a liquid feed conduit, the other beingconnected to a liquid discharge conduit having adjustable valve meanstherein, a plurality of baffled disposed obliquely with respect to saidinterior surface of said shaft so that, in combination, said baffleswill be contacted by all liquid and all granules passing down throughsaid shaft.
 2. The apparatus as claimed in claim 1, wherein saidvertical shaft is provided with a double wall extending along the axiallength of said shaft from a point proximate the liquid discharging meansto a point below said baffles, said double wall being provided withconduit means for the supply and discharge of a heating agent.
 3. Theapparatus as claimed in claim 1, wherein a mixing vessel is provided toreceive the material discharged from the lower end of said verticalshaft, said mixing vessel comprising a sloping, rotary drum having asupply opening in its upper face and a discharge opening in its lowerface, and having vanes disposed interiorly thereof.